A two-dimensional electron system is coherently polarized and vibrationally excited by means of an optical phonon or photon corresponding to inter-sub-band energy. This vibrational quantum is called two-dimensional electron plasmon. At a typical two-dimensional electron density (1011 to 1012 cm−2), the two-dimensional electron plasmon is of a wavelength range of micron to submicron, and the basic mode frequency reaches the terahertz band. Therefore, the two-dimensional electron plasmon has a capability of serving as a mechanism for realizing functions of electromagnetic wave oscillation, electromagnetic wave detection, frequency mixing, frequency multiplication, etc. in the terahertz band. Studies on the physical properties of the two-dimensional electron plasmon were started in the early 1970's. Studies on application to terahertz devices were started in the 1990's; the history is still short, and practical elements have not yet been developed.
M. Dyakonov and M. Shur proposed a terahertz band application of two-dimensional electron plasmon in a high electron mobility transistor (HEMT) structure (see Non-Patent Document 1). Since electron density, which determines the plasmon resonance frequency, can be controlled by means of gate bias, a frequency-variable characteristic which is of practical importance can be realized. The boundary conditions of the source and drain can be made asymmetric by means of a difference in bias dependency between the gate-source capacitance and the gate-drain capacitance. A radiation mode electromagnetic wave can be taken out at the drain open end. An odd-order harmonic component can be taken out from the drain end, and an even-order harmonic component can be taken out from the vicinity of the center of the channel. Therefore, when the density of two-dimensional electrons is modulated by the frequency difference terahertz component of photoconductive electrons generated through interband excitation by two light wave photons, plasmon resonance in the terahertz band can be induced (see Non-Patent Documents 2 and 3). This two-dimensional electron plasmon resonance wave is of a non-radiation mode, and cannot be radiated to the outside. However, through provision of a metal grating or antenna structure in the vicinity of the two-dimensional electron plasmon, the terahertz-band two-dimensional electron plasmon vibration of the non-radiation mode can be converted to a radiation mode electromagnetic wave (see Non-Patent Documents 3 and 4). Thus, a terahertz-band photo mixer utilizing the two-dimensional electron plasmon can be realized.
When a photo mixer is configured, the following two points are important.    (1) The efficiency of conversion from light waves to two-dimensional electron plasmon resonance.    (2) The efficiency of conversion from two-dimensional electron plasmon resonance to radiation electromagnetic waves.Of these, the efficiency of conversion from two-dimensional electron plasmon resonance to radiation electromagnetic waves mentioned in (2) above, which is the object of consideration in the present invention, will be described from the viewpoint of progress in conventional techniques.
A grating coupler has been introduced as a mode conversion mechanism for converting a two-dimensional electron plasmon resonance wave of the non-radiation mode to a radiation mode electromagnetic wave. This is well known as the so-called Smith-Purcel effect.
As disclosed in Non-Patent Document 5, R. J. Wilkinson, et. al. formed the gate electrode in the shape of a nested double grating, periodically modulated the density of two-dimensional electrons, and observed far-infrared-light transmission and reflection characteristics. They asserted that the grating structure formed by means of two-dimensional electron density modulation functions as a photo coupler which efficiently absorbs far-infrared light, and the plasma resonance frequency can be controlled by means of two-dimensional electron density modulation.
In Non-Patent Document 4, S. A. Mikhailov explains electromagnetic-wave propagation characteristics by making use of the structural parameters of this grating structure, and physical property parameters of the material, including density of two-dimensional electrons, electron density at the grating portion, drift speed of electrons, and scattering relaxation time. He also showed that when the plasma frequency determined as a result of the density of two-dimensional electrons being periodically modulated by the grating becomes equal to the plasma frequency of the grating itself; if the degree of scattering of electrons is low, the transmission coefficient of electromagnetic waves exceeds 1 in a range in the vicinity of and lower than the plasma frequency, and an amplification gain can be obtained. As a specific measure, there was proposed to introduce, in place of a metal grating, a quantum wire whose conductivity is as low as that of the two-dimensional electron plasmon.
As described in Non-Patent Document 6, X. G. Peralta, et. al. formed two two-dimensional electron layers, periodically modulated the density of two-dimensional electrons of the upper layer by means of a single-grating-type gate, and observed the characteristic of optical response of the two-dimensional electron layers to radiation of terahertz electromagnetic waves. They showed that the resonance property which influences the optical response characteristic at the plasma frequency determined as a result of subjection to periodical density modulation is enhanced through provision of two two-dimensional electron layers.
Another example of instruction of a grating structure for mode conversion is introduction of a spiral antenna structure which was proposed by V. Ryzhii, et. al. in Non-Patent Document 3.    Non-Patent Document 1: M. Dyakonov and M. Shur, Phys. Rev. Lett., 71(15), 2465 (1993)    Non-Patent Document 2: T. Otsuji, Y. Kanamaru, et. al., Dig. the 59th Annual Dev. Res. Conf., Notre Dame, Ind., 97(2001)    Non-Patent Document 3: V. Ryzhii, I. Khmyrova, and M. Shur, J. Appl. Phys., Vol. 91, No. 4, 1875 (2002)    Non-Patent Document 4: S. A. Mikhailov, Phys. Rev. B, Vol. 58, pp. 1517-1532, 1998    Non-Patent Document 5: R. J. Wilkinson, et. al., Journal of Applied Physics, Vol. 71, No. 12, pp. 6049-6061, 1992    Non-Patent Document 6: X. G. Peralta, et. al., Applied Physics Letters, Vol. 81, No. 9, pp. 1627-1629, 2002